1. Field of the Invention
The present invention relates to a photosensitive resin composition which is useful for the production of a flexographic printing plate because of its rubber elasticity and good ink resistance and its capability of being developed with an aqueous system. The present invention also relates to a printing plate stock made with said photosensitive resin and a finished printing plate thereof.
2. Description of the Prior Art
Photosensitive resin-based flexographic printing plates hitherto known are designed to be developed with an organic solvent, which poses a problem associated with toxicity and fire hazards and environmental pollution. To address this problem, there have been proposed new photosensitive resin compositions capable of development with an aqueous system, as enumerated below.
(1) One containing a copolymer, a photopolymerizable unsaturated monomer, and a photosensitizer, said copolymer being composed of a conjugated diene hydrocarbon, an .alpha.,.beta.-ethylenic unsaturated carboxylic acid (or a salt thereof), and a monoolefinic unsaturated compound. (Japanese Patent Kokai Nos. 134655/1977, 10648/1978, and 22339/1986)
(2) One containing a polymer of conjugated diene hydrocarbon or a copolymer of conjugated diene hydrocarbon and monoolefinic unsaturated compound, a hydrophilic polymeric compound, a non-gaseous ethylenic unsaturated compound, and a photopolymerization initiator. (Japanese Patent Kokai No. 211451/1985)
(3) One containing a hydrophobic oligomer with an .alpha.,.beta.-ethylenic unsaturated group, a water-swellable elastomeric substance, and a photopolymerization initiator. (Japanese Patent Kokai No. 1703055/1985)
There have also been proposed other new photosensitive resin compositions as enumerated below.
(1) One containing hard organic fine particles, which is intended for the improvement of printing plates in mechanical strength and impact resilience. (Japanese Patent Kokai No. 8648/1988)
(2) One having a two-phase structure of continuous phase and dispersed phase, the former containing a diazo compound and dichromate and the latter containing particles smaller than 10 .mu.m in size. This is intended for the improvement of printing plates in ink receptivity. (Japanese Patent Publication No. 36731/1984)
The former one is capable of development with an aqueous developing solution such as aqueous alkaline solution or with a developing solution composed of an aqueous alkaline solution and an organic solvent, while it can hardly be developed with tap water or pH 5.0-9.0. Moreover, it gives rise to a relief part which lacks a sufficient ink resistance.
The latter one also has a disadvantage. For the composition to be capable of development with an aqueous system, it should contain a hydrophilic component in the continuous phase, and the amount of the hydrophilic component should be larger than that of the component forming the dispersed phase from the standpoint of thermodynamic safety. This leads to an incompatibility of the water-developing performance with the ink resistance of relief parts.
Basically, a photosensitive resin composition should have good light transmission properties and hence should be composed of those components which are highly compatible with one another. This requires that the hydrophilic polymer, which originally has a high polarity in many cases, should be mixed with other components which also have a high polarity. However, this compatibility is not realized in the case where the composition contains a polymer of conjugated diene hydrocarbon, which usually has a low polarity. (In other words, the hydrophilic polymer to be used is limited in its kind and amount.)